Present-day light water nuclear reactors, both pressurized water reactors and boiling water reactors, require fuel to be installed in a reactor pressure vessel prior to operation. The fuel is often in the form of a fuel assembly, where individual fuel rods, which contain a series of parallel arranged rods each containing stacked pellets, are held together by a structural skeleton arrangement.
After the reactor pressure vessel is operated and the nuclear fuel in the fuel assemblies is depleted, the depleted fuel assemblies must be removed and replaced with new fuel assemblies. Additionally, non-depleted fuel assemblies must be moved to other areas of the reactor core and inspected for defects prior to restarting the reactor. The moving of the fuel assemblies, both depleted and non-depleted, must be performed in a safe and controlled manner as dropping of a fuel assembly could lead to damage of nuclear fuel and potential release of radioactive material.
Detailed visual and radiographic inspections of fuel assemblies and special lifting devices (devices that move the fuel assembly) help prevent load lifting accidents due to degraded structural components. It has been found that after exposure to radiation, some fuel assemblies exhibit stress corrosion cracking of the upper guide thimble sleeves which attach the top nozzle of the fuel assembly to the guide thimbles.
Industry experience has also found that current devices and methods to lift structurally damaged fuel assemblies are costly for plant operators, while not eliminating the potential for load lifting accidents from problem prone areas such as failure of the top nozzle to guide thimble connection from stress corrosion.
If a fuel assembly exhibits stress corrosion cracking or some other structural defect, current practices require underwater disassembly of the fuel assembly and removal of the individual pieces or establishment of additional structural corrections to the damaged fuel assembly to allow an adequate transfer of the structural load of the assembly. Both of these repair alternatives are expensive and require significant structural analysis of the damaged fuel assembly to ensure a safe load lift.
There is a need for a fuel assembly lifting device and method which can be used with existing fuel assembly configurations in nuclear reactors, both pressurized and boiling water reactors.
There is a further need to provide a fuel assembly lifting device and method which will allow for safe movement of a fuel assembly when the fuel assembly has stress corrosion cracking of the upper guide thimble sleeves that attach the top nozzle to the guide thimbles.
There is still further need for a device which will properly interface with existing fuel handling equipment and storage racks for cost efficient effective movement of fuel assemblies.